Thunderbird provides many keyboard shortcuts and some action buttons. See:
https://support.mozilla.org/en-US/kb/keyboard-shortcuts
My guess is that you accidentally hit the "
a" key or
Archive button, causing that message to be archived. Archiving is a feature of Thunderbird - see:
https://support.mozilla.org/en-US/kb/archived-messages
The navigation to the
Account Settings screen is through a couple of methods:
- (hamburger three horizonal lines button) > Options > Account Settings
- (click the email address for one of your accounts in the left pane) > Accounts section > View settings for this account
As shown in the help link above, the
Copies & Folders section is used to control the Message Archives feature. By default the archive feature is enabled. if you uncheck the "
Keep message archives in:" checkbox, the archive feature will be disabled for that account, and the Archives button and "a" shortcut will not be available.
A folder named Archive or Archives is present by default on some (but not all) email accounts. For example, Fastmail originally did not use such a folder as standard, but now it's a standard system folder on new accounts and it cannot be deleted.
Also, I believe that Thunderbird by default will create such a folder on the email server if IMAP is used and you use the "a" shortcut or Archive button if the Archives feature is enabled.
Even though you may not be able to delete the Archive (or Archives) folder from the email server, you can remove it from view in Thunderbird by
unsubscribing from that folder. Use the
Advanced Features > Manage folder subscriptions section of the screen you see when you click the address of the account in the left pane (or use the
hamburger button > File > Subscribe... feature).
Bill